Green’s Dictionary of Slang

a small watchdog.

1811		Lex. Balatronicum n.p.: Gnarler. A little dog that by his barking alarms the family when any person is breaking into the house.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).
1895		Crowe Aus. Sl. Dict. 31: Gnarlers, little dogs.